This invention relates to a Class "B" amplifier circuit of the type which uses a Class "B" converter to obtain the desired mode of operation.
Typical prior art Class "B" type amplifiers are shown in U.S. Pat. Nos. 3,573,645 (FIG. 3) and 3,786,364 (FIG. 4). While these amplifier circuits provide Class "B" operation, they suffer from a number of drawbacks. Since the two prior art circuits are basically similar as they relate to the present invention, the more basic circuit shown in FIG. 3 of U.S. Pat. No. 3,573,645 will be described. In FIG. 3, an AC input signal is provided to a single-ended low impedance input node at the junction of the emitter of transistor 11 and the base of transistor 12. The input signal is processed by a push-pull phase splitter composed of transistors 11 and 12 and their associated circuitry (as shown in FIG. 1 of the patent). The outputs of this push-pull phase splitter are then coupled to an output stage which operates in a Class "B" mode. However, a number of disadvantages result from applying the AC input signal directly to the phase splitter and then using the phase splitter output to drive an output stage operating in the class "B" mode.
First, this configuration provides a low input inpedance for the AC signal, which may present problems in certain applications. Furthermore, in order to achieve the necessary current gain in the output stage as required by this configuration, the prior art circuit of FIG. 3 employs a current amplifier configuration in which there are two base-emitter junctions (of transistors 28 and 25) connected between one output of the phase splitter and the circuit output terminal 27. This results in a voltage drop of approximately 1.4 volts between the phase splitter and the output terminal of the circuit, which serves to limit the dynamic range of the circuit. Since this voltage drop remains substantially constant regardless of the power supply voltage used, it will be seen that this problem becomes increasingly significant in low-voltage applications, where the voltage drop due to the series-connected base-emitter junctions may constitute a significant portion of the total dynamic range available.
In FIGS. 4 and 5 of U.S. Pat. No. 3,852,676 there are shown circuits which superficially resemble the Class "B" converter portion of the circuit of FIG. 1 of the present invention. However, these prior art circuits are detector circuits, rather than Class "B" converters for use in amplifiers. The prior art circuits are used standing alone, and there is no suggestion in the accompanying specification that the detector circuits disclosed therein could be used as part of a Class "B" amplifier. Furthermore, the detector circuits of FIGS. 4 and 5 incorporate an RC time constant (R5-C2) which serves to filter or smooth the output signal, thus making the circuits of U.S. Pat. No. 3,852,676 totally unsuitable for use in the present invention.